In mobile communications, a wideband wireless channel is usually frequency-selective so that deep fade occurs in some portions of the channel bandwidth, making these portions not preferable for use in data transmission. Therefore, adaptive allocation of frequency resources of the channel for data transmission is usually employed in mobile communication systems. Performing this allocation requires a knowledge of the channel quality.
As an example, FIG. 1 depicts the channel-quality information received by a base station (BS) from a user equipment (UE) in a LTE system, where the BS employs this information to perform frequency-resource allocation. The BS first sends a reference signal to the UE via a downlink channel. Based on the received reference signal, the UE estimates a frequency response 110 of the channel over the bandwidth. In the LTE system, the channel bandwidth is partitioned into bandwidth parts each having one or more subbands. Each subband is subdivided into resource block groups (RBGs) each having a certain bandwidth. Based on the frequency response 110, the UE computes a channel-quality indicator (CQI), as defined in the LTE specification, for each subband. The UE also computes a wideband CQI 120 representing an average CQI over the channel bandwidth. The UE feedbacks the channel-quality information via an uplink channel, which may be different from the downlink channel in frequency. In order to save radio resources of the uplink channel, the LTE specification specifies a limited amount of channel-quality information to be sent to the BS. The UE only sends the wideband CQI 120 and CQIs 130a-d of selected subbands 135a-d, where each of the selected-subband CQIs 130a-d is the highest CQI among the one or more subbands of one bandwidth part. The BS is required to allocate the frequency resources of the downlink channel based on the limited channel-quality information.
Although one simple allocation strategy is to use only the selected subbands reported by the UE, as is taught by US20140098663, it is more desirable if the BS can estimate CQIs of all the subbands before frequency-resource allocation. US20120327874 suggests a method to estimate CQIs of all the subbands by having that within one bandwidth part, the CQI of the selected subband therein (e.g., the CQI 103b of subband no. 6 of bandwidth part no. 2) is retained while CQIs of remaining subbands (subbands no. 5 and 7) are assigned a value of the wideband CQI 120 plus or minus a certain margin. This method assumes flatness of the estimated CQIs in non-selected subbands over a bandwidth part. However, this approach leads a large error between the measured CQI and the estimated one in the presence of a large roll-off in CQI, such as the large roll-off 142 in the frequency response 110 shown in FIG. 1.
There is a need in the art for a technique used in the BS to estimate CQIs of all subbands based on the limited channel-quality information returned by the UE while the large roll-off problem can be alleviated. The estimated CQIs of all the subbands are useful for adaptive allocation of frequency resources of the channel.